Cardiovascular disease is a very serious threat to life and health in the U.S. It accounts for one-half of all deaths in the U.S.
Coronary artery disease is recognized as the leading cause of heart disease. It is the principle cause of death after age of 40 in men and after age of 50 in women.
Coronary artery disease kills and disables people in their most productive years. It accounts for $8.6 billion spent in 1981 for medical care. Coronary artery disease causes about 800,000 new heart attacks each year and an additional 450,000 recurrences.
It is estimated that a 30-year old American male would survive to age 79 rather than 73 if coronary artery disease could be eliminated. In 1982, there were 640,000 coronary deaths in the U.S. In the age range 35 to 64 about 75% of all cardiac deaths are due to coronary artery disease. Sudden, unexpected, out of hospital coronary deaths that occur too rapidly to allow arrival at the hospital while the patient is still alive account for more than one-half of all coronary fatalities.
Examination of the incidence, prevalence, mortality, and history of coronary artery disease suggests the need for a preventive approach. Correction of predisposing factors and innovative advances in diagnosis and therapy can make a major impact at least in reduction of coronary artery disease. This is essential since when a region of the myocardium, the heart muscle, is irreversibly damaged no current therapy can be expected to restore full heart function.
Early detection of coronary artery disease is important in reducing the extent of myocardial injury. Modern techniques of cardiology such as coronary angiography, thallium perfusion imaging, echocardiography, and radionuclide ventriculography permit highly sensitive and specific tests of myocardial ischemia (reduced blood supply to myocardial tissue), myocardial infarction (complete lack of blood supply causing death of myocardial tissue), and wall motion abnormalities secondary to coronary artery disease.
Unfortunately, the above noted methods are either invasive or expensive. As a result, ordinary electrocardiography and analysis of the resulting electrocardiograph (ECG) remains a very widely used, noninvasive tool in preliminary diagnosis of myocardial ischemia. Evidence of coronary artery disease is frequently identified by changes in the configuration of a certain portion of the ECG signal designated as the ST-segment. These changes manifest themselves clearly when the heart is under stress. The sensitivity and specificity of the ECG in detection of coronary artery disease can be increased by simultaneously stressing the heart.
There are several forms of stressing the heart which can be used to assess patients with chronic ischemic heart disease. These include dynamic exercise, isometric exercise, pharmacological stress, and atrial pacing.
For the past 50 years, the results of using exercise induced stress in combination with a simultaneously recorded ECG for both diagnostic and prognostic purposes have been the subject of intense research. However, not all patients are able to exercise. This can be due to obesity, poor physical condition, neuropathy, respiratory limitation, claudication, arthritis, paraplegia, lower limb amputation, diabetes, unstable angina, or risk of complication and physical incapacity in patients with recent myocardial infarction.
Cardiac stress induced by isometric exercise is often inadequate in provoking ischemic events. Pharmacological stress induced by intravenous drugs such as dipyridamole or dobutamine is commonly associated with cardiac or non-cardiac side effects, unknown pharmacokinetics for individual patients, and long delays in taking full effect. It is also ineffective in eliciting an adequate electrocardiographic response.
Transesophageal atrial pacing-induced stress in conjunction with two-dimensional echocardiography or radionuclide ventriculography has been reported to be a safe and accurate method in diagnosing ischemia. It is especially useful in patients who cannot perform an adequate exercise stress test. Iliceto S., Sorino M., D'Ambrosio G., et al. "Detection of Coronary Artery Disease By Two-Dimensional Echocardiography and Transesophage Atrial Pacing", J. Am Coll. Cardiol, 5(5):1188-97,1985.
ST elevation in esophageal ECG has also been shown by Kates R.A., Zaiden J.R., and Kaplan J.A. "Esophageal Lead For Intraoperative Electrocardiographic Monitoring", published in Anesth. Analog., 61:781-5, 1982.
Transesophageal pacing offers the advantages of direct control over the heart rate and an increased control over the degree of myocardial stress which is noninvasive. Further, it does not depend on the physical condition of the patient, and is not subjected to the wide variability in heart rates and blood pressure responses commonly associated with dynamic exercise.
The esophageal route also provides a vantage point to detect posterior ischemia. The mortality rate associated with posterior abnormalities has been estimated at 15%. Electrocardiographic diagnosis of posterior abnormalities such as ischemia is often difficult or equivocal. This is because no surface lead records the electrical activity of the posterior cardiac wall directly.
Under the best of conditions, the posterior wall of the left ventricle is hidden from the chest electrodes by the anterior wall. Electrodes located on the back of the patient are not of much use because of their distance from the heart and because of the intervening high resistivity lungs.
In contrast to the body surface, the esophagus provides a vantage from which to view the posterior aspects of the heart at close range and without intervening active or resistive tissue. Studies have shown that the esophageal ECG recorded at the ventricular level is as specifically diagnostic of posterior myocardial abnormalities as the precordial ECG is diagnostic of anterior wall abnormalities. Hamilton J.G.M., Nyboer J., "The Ventricular Deflections in Myocardial Infarction: An Electrocardiographic Study Using Esophageal and Precordial Leads", Am. Heart J., 15:414-25, 1938.
Nevertheless, the use of the esophageal ECG has not become popular for at least two reasons. First, all studies reported to the present time have used an electrode mounted at the end of a stomach tube, with considerable discomfort to the patient. Second, excessive amounts of baseline variation are present, due to the esophageal motion produced by respiration, peristalsis and cardiac contraction. These variations make it difficult to make accurate measurements, of small ST-segment shifts associated with ischemia.
Computer implementation of automatic detection of the ST-segment changes in the surface ECG and the esophageal ECG is highly desired. The goal of such a system is to provide an operator independent, reliable, and reproducible tool to aid clinicians in detection and management of myocardial ischemia of the total heart (anterior and posterior surfaces). Several algorithms have previously been implemented in computerized electrocardiographs to analyze the surface ECG during an exercise stress test. However, these methods have been ineffective in processing the unique surface ECG recorded during a transesophageal pacing procedure. This is because the presence of large pacing artifacts alters the shape of the sensed signals and complicates computerized ECG analysis.
Furthermore, to date, there has been no algorithm usable to analyze the esophageal ECG for detection of posterior ischemia. This has mainly been due to the technical difficulties that have been associated with noninvasive, high quality recording of the esophageal ECG, especially during transesophageal pacing.
Thus, there continues to be a need for apparatus and methods which make possible the processing of the unique surface ECG which can be recorded during a transesophageal pacing procedure. Preferably, such an apparatus and method will be relatively inexpensive and at the same time effective to block generated artifacts from interfering with the operation of the electrocardiograph.